Most ridge vent options are fairly similar in that they are a vented foam material with a high net free area. Net free area is simply the amount of space available for air to freely flow. Vented materials are typically made of an open cell material, meaning they are less dense and have the ability to allow airflow through the material, while also sealing the roof off from the elements, insects and other things that may come from outdoors.
It is important for the net free area at the ridge cap of the roof to be equal to or slightly less than the net free area at the eaves of the roof. This is important to create a positive airflow from the eaves and out trough the ridge. If the net free area at the ridge vent is larger than at the eaves, this can actually result in a negative airflow, and hot air will remain trapped under the roof.
Where the differences come in vented materials is of course in the quality of the material. It is important for vented materials to be weather and UV resistant, otherwise the material can degrade rather quickly once put under prolonged exposure to the outdoors, compromising the venting system.
If a metal roof does not use ventilation at the ridge and eaves of the roof, the other options would be gable roof vents, round gravity vents or power vents. While ventilation at the ridge and eave of the roof is a commonly used method for ventilation, these other methods are effective for any kind of roofing that does not provide air intake at the eave and exhaust at the ridge.